With the configuration field Use ticker rate you can select which values of the exchange tickers should be used.
Highest bid / Lowest ask
When selected the highest bid or lowest ask value of the exchange ticker is used.
Use last if higher/lower
When selected the last price on the exchange ticker will be used instead of the highest bid or lowest ask value.
Always use last tick
When selected the last price on the exchange ticker will always be used. Highest bid and lowest ask will be ignored.